INTERFERENCE
When two waves traveling through a material meet each other, they do not affect each other, so the oscillations of a point will be equal to he sum of the affect of the two waves individually. In the diagram shown opposite the, waves are in phase and add up to give a larger displacement than either would on its own. This is constructive interference, if the waves where π out of phase they would have a lower displacement (zero if their amplitudes where equal), this is destructive interference.